System for positioning a surgical device

ABSTRACT

The invention relates to a system for positioning a surgical device, said surgical device comprising:
         a base ( 1 ) designed to be held in a user&#39;s hand,   an end-effector ( 2 ) for mounting a surgical tool, respectively a guide ( 20 ) for guiding a surgical tool ( 3 ), said surgical tool being designed to treat a planned volume of a part (P 1 ) of a patient&#39;s body,   an actuation unit ( 4 ) connected to said base ( 1 ) and said end-effector ( 2 ) for moving said surgical tool ( 3 ), respectively tool guide ( 20 ), with respect to the base ( 1 ) in order to treat said planned volume,   said positioning system comprising:       (i) a tracking unit ( 200 ) configured to determine in real time the pose of at least one of the tool ( 3 ), the end-effector ( 2 ) and the base ( 1 ) with respect to the part to be treated,   (ii) a control unit ( 300 ) configured to:
       (a) compute in real time the working space of the surgical tool for said determined pose,   (b) compute in real time the volume that remains to be treated to achieve the treatment of the planned volume,   
       (iii) a user interface ( 400 ) coupled to the control unit ( 300 ) and configured to display together:
       the planned volume, said planned volume comprising a volume (p 2 ) that remains to be treated and, if applicable, an already treated volume (p 1 ), and   the working space (p 3 ) of the surgical tool.

FIELD OF THE INVENTION

The invention relates to a system for positioning a surgical device.

BACKGROUND OF THE INVENTION

Robotic devices have been used in surgery since the late 1980s,beginning with a serial kinematic structured industry robot programmedto position a tool guide at a specified location near the head [Kwoh1988] [Lavalée 1989]. During the 1990s, robots were introduced intoorthopedic surgery with the ROBODOC system (ISS, USA) and the CASPAR(U.R.S., Germany) for hip and knee surgery [Bargar 1998, Prymka 2006].However, not only did these autonomous systems not show more long termadvantages than conventional techniques, they also showed longeroperation times and increased blood loss [Bach 2002]. One of thedisadvantages of these systems was the rigid fixation of the bone to therobot and the fully autonomous processing which took control away fromthe surgeon. Autonomous robots are not well-suited for soft-tissuesurgery, as the shape of tissue may change when cut or pushed, or as aresult of the patient's breathing.

Master-slave controlled robots, controlled via a console and remotevisual feedback, have also been introduced [Maeso 2010]. The da Vincisystem is used in minimally-invasive laparoscopic abdominal surgery,where the surgeon controls up to four robotic arms and the movements ofthe surgeon's hand can be filtered and scaled to enable preciseinstrument micro-movements.

In contrast to the da Vinci approach, where the surgeon controls therobot from a distance using a remote console, robots have also beenintroduced for cooperative work. These systems either position a toolguide and the surgeon itself guides the instrument [Liebermann 2006,Plaskos 2005], or the surgeon guides a haptic-controlled robot, and therobot prevents access to forbidden areas.

The concept of the haptic-controlled robot using “active constraints” or“virtual fixtures” was first implemented by the Acrobot system [Davies2007, Yen 2010], and later by the MAKO Surgical Corp. RIO system. forUKA and THA [US 2006/0142657 A1 (Quaid et al.), Lonner 2010, Dorr 2011].A randomized prospective study of the Acrobot system showed that withrobotic bone preparation in UKA, the tibiofemoral alignment was within2° of the planned position, whereas in the control group only 40% werebelow 2° [Cobb 2006].

A similar approach, but without using a huge robot system, is the use of“intelligent” high speed burrs that can be programmatically enabled anddisabled in pre-planned areas. Whereas the Navigated Control conceptcontrols the rotating speed of the burr [Strauss 2005], the PrecisionFreehand Sculptor comprises a burr that retracts behind a guard [Brisson2004, WO 2011/133927 A2 (Nikou et al.)].

US 2005/0171553 (Schwarz et al.) discloses a handheld device fortreating a body part that comprises a base, a tool that is able to movewith respect to the base, and an actuation unit to move the tool withina predetermined working space to a predetermined position on the part tobe treated. This device takes into account the movements of the base andof the part to be treated by detecting the position of the tool and theposition of the part to be treated, by comparing said positions with atarget position and by adapting the actuation unit accordingly. US2012/0143084 (Shoham) describes a handheld device for treating a bodypart that comprises a handle, a tool that is able to move with respectto the handle, and a robot to move the tool within a predeterminedworking space to a predetermined position on the part to be treated.This device is able to detect the position of the tool with respect to aforbidden zone and to change the pose of the robot if the user moves thehandle by an amount that would bring the tool within said forbiddenzone. A disadvantage of these hand-guided tools is that the milling pathitself needs to be controlled by the surgeon, which may result innon-efficient bone removal, inaccurate milling surfaces and unintendedheat emission. In addition, significant time is required to obtainreasonable overall accuracy. Locally, milling surfaces are always bumpyor irregular, and to compensate for such bumps, cement is usuallyrequired between the milling surface and the implant, instead ofnon-cemented implants which are often preferred. Even with cement, theend result of such a process is an overall loss of accuracy.

For applications where no tremor is permitted, such as retinal surgery,a steady hand manipulator has been introduced [Mitchell 2007, Uneri2010] as well as the handheld actively stabilized Micron device [Becker2011, MacLachlan 2012].

US 2011/0208196 (Radermacher et al.) discloses a handheld reactivedevice for creating and applying constraints to the user that comprisesa handle, a tool that is able to move with respect to the handle, and asupport element connected to the handle by means of which the handle maybe supported on a body surface. The support element is movable via anactuation unit which enables tool repositioning with respect to bodysurface by shifting the support element, based on sensor data obtainedduring the treatment. However, this device is applying constraints tothe user depending on the material being worked, and it is not able tooptimize the tool path actively in view of treating a planned volume ofthe body.

There remains a need for a lightweight, handheld surgical device whichenables tool path optimization and compensation of the surgeon's smallmovements to minimize vibration and optimize accuracy.

BRIEF DESCRIPTION OF THE INVENTION

Accordingly, the invention provides a System for positioning a surgicaldevice, said surgical device comprising:

-   -   a base designed to be held in a user's hand,    -   an end-effector for mounting a surgical tool, respectively a        guide for guiding a surgical tool, said surgical tool being        designed to treat a planned volume of a part of a patient's        body,    -   an actuation unit (4) connected to said base and said        end-effector for moving said surgical tool, respectively tool        guide, with respect to the base in order to treat said planned        volume,    -   said positioning system comprising:        (i) a tracking unit configured to determine in real time the        pose of at least one of the tool, the end-effector and the base        with respect to the part to be treated,        (ii) a control unit configured to:    -   (a) compute in real time the working space of the surgical tool        for said determined pose,    -   (b) compute in real time the volume that remains to be treated        to achieve the treatment of the planned volume,        (iii) a user interface coupled to the control unit and        configured to display together:    -   the planned volume, said planned volume comprising a volume that        remains to be treated and, if applicable, an already treated        volume, and    -   the working space of the surgical tool.    -   By “volume to be treated” is meant, in the present text, either        a one-dimensional (1D) volume that extends along a line (e.g.        when the treatment consists in drilling a bore in the part), a        two-dimensional (2D) volume that extends along a plane (e.g.        when the treatment consists in sawing the part), or a        three-dimensional (3D) volume (e.g. when the treatment consists        in removing a predefined volume to match an implant perfectly or        in removing a precise part that has been defined on images        registered with the working coordinate system).

By “partial mechanical link” is meant a mechanical link between at leasttwo parts, wherein a relative movement of said at least two parts in atleast one degree of freedom is possible. This term excludes a “complete”mechanical link, i.e. a link wherein no relative movement between theparts is allowed (an example of such complete mechanical link would beattaching the base rigidly to the part to be treated (e.g. a bone) by atleast one screw).

As described in further detail below, when a support unit is used, saidpartial mechanical link provided between the base and the part of thepatient's body may be direct, meaning that the support unit is incontact with the part to be treated itself, or indirect, meaning thatthe support unit is in contact with a part of the patient's bodyadjacent to the part to be treated. Said adjacent part may consist of abone belonging to the same joint as the part to be treated, or of softtissues that surround the part to be treated. An indirect partialmechanical link may also be obtained when the support unit is held by auser's hand and that said hand leans onto the part to be treated or thesoft tissues and skin surrounding the part to be treated.

Depending on the part with which the support unit makes contact and onthe design of the support unit itself, said partial mechanical link maybe rigid or damped.

Such a device is able to compensate for a given amount of pose errors(e.g. due to small movements of the user).

By “pose” is meant, in the present text, the 3D position and 3Dorientation of a tool in up to six degrees of freedom. It is to be notedthat depending on the application, it may not be necessary to determineall six degrees of freedom but only one or some of them.

By “path” is meant a set of tool poses that allow the planned volume tobe treated.

A path is “optimized” if said set of poses is computed based on currentrelative poses of the device and of the part that remains to be treated,so as to minimize at least one of the following:

-   -   number of necessary repositioning actions of the device to        achieve the treatment;    -   time needed to treat the planned volume;    -   heat generated by the tool;    -   surface roughness and accuracy of the treated part;    -   orientation of the tool (or of cutting edges of a milling tool)        with respect to the bone surface;    -   (this list is not limitative).

According to advantageous but optional embodiments of the invention,taken alone or combined:

-   -   the user interface is configured to display the planned volume        together with the surface of the part to be treated,    -   the user interface is configured to display the working space as        an overlay on the planned volume,    -   the control unit is configured to compute the intersection of        the volume that remains to be treated and the working space of        the surgical tool and the user interface is configured to        display said intersection,    -   the user interface is configured to display in real time the        pose of the tool with respect to the planned volume,    -   the user interface is configured to display an indication on how        the base of the device should be repositioned,    -   the system comprises a planning system configured to determine a        volume to be treated by the tool and, if appropriate, at least        one processing parameter of the tool;    -   a drill guide or a saw guide is comprised in the end-effector or        in the support unit and the tool, which may or may not be a part        of the device, is a drill or a saw that is intended to be moved        within the axis or the plane of said guide;    -   the surgical tool comprises a saw, a drill, a mill, a shaver or        a burr;    -   the tracking unit comprises at least one emitter and at least        one sensor, said at least one emitter being mounted on the base        or on the end-effector;    -   at least one sensor of the tracking unit is adapted to be        mounted on the part to be treated.

The above-described system may be operated according to the processdescribed below.

A handheld device is provided to a user, said handheld devicecomprising:

-   -   a base suited to be held in the user's hand,    -   an end-effector for mounting a surgical tool, respectively a        guide for guiding a surgical tool, said surgical tool being        designed to treat a planned volume of a part of a patient's        body,    -   an actuation unit connected to said base and said end-effector        for moving said surgical tool and/or tool guide with respect to        the base in order to treat said planned volume.

For a given status of the volume to be treated, before starting the nexttreatment one possibility is, that first an optimal starting pose forthe base is calculated. A starting pose may be optimized concerning thefollowing criteria:

-   -   maximum reachable part of the remaining volume to be treated    -   maximum distance of the remaining volume to be treated to the        limits of the robot workspace (if the entire volume is inside of        the reachable robot workspace). The distance is important as to        have maximum space where the robot can compensate for movements        of the robot base.    -   Orientation of the tool relative to the bone surface    -   (the list is not limitative)

The user is then guided by the user interface to position the robot basein the optimal pose. Therefore different graphical illustrations can beused as targets (lines, bars, crosses or 3D illustrations) to help theuser positioning the base at the optimal starting pose. In general, thestarting pose is a not represented by a single value but by a range ofadmissible values and the target is then represented by a large area andnot a precise geometric element.

If applicable, the user puts the support unit in contact with the partto be treated or a region of the patient's body adjacent to the part tobe treated so as to provide a partial mechanical link between the base,respectively the end-effector, and the part to be treated.

The pose of at least one of the tool, the end-effector and the base withrespect to the part to be treated is determined in real time by atracking unit.

A control unit implements the following process:

-   -   (a) compute in real time an optimized path of the end-effector        with respect to the base depending on said measured pose,    -   (b) detect whether said computed path of the tool, respectively        of the end-effector, can be achieved without changing the pose        of the base, and, in the negative, determine a possible        repositioning (new optimal starting position) of the base with        respect to the part to be treated,    -   (c) configure the actuation unit so as to move the end-effector        according to said computed path, and    -   (d) iterate steps (a) to (c) until the planned volume has been        treated.

During operation of the tool, a user interface provides feedbackinformation to the user; in particular, in an advantageous embodiment,the user interface indicates whether said computed path is achievablewithout changing the pose of the base and/or the support unit withrespect to the part to be treated, and, in the negative, indicates apossible repositioning of the base and/or of the support unit determinedby the control unit.

When a repositioning is necessary, it can be as for the startingposition such that the robot and the tool are stopped and the user isguided until he reaches the optimal position and restarts the system, orit can be a continuous process such that during the repositioning intothe direction of the new optimal position the process is not stopped.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features, embodiments and advantages of the invention will beapparent from the detailed description that follows, based on theappended drawings wherein:

FIG. 1 shows an overview of a surgical system according to theinvention;

FIG. 2A to 2K illustrate planning details that can be displayed on ascreen connected to the control unit;

FIGS. 3A to 3L shows examples of embodiments of the handheld device andof the support unit;

FIGS. 4A to 4E show devices with different implementations of thetracking unit;

FIGS. 5A to 5C show different embodiments of the user interface;

FIG. 6 is a flow chart representing a way of implementing a systemaccording to the invention;

FIGS. 7A and 7B show an embodiment of a handheld device suited for uniknee arthroplasty;

FIGS. 8A to 8C show an embodiment of a handheld device suited for totalknee arthroplasty;

FIGS. 9A and 9B show an embodiment of a handheld device suited for thetreatment of femoro-acetabular impingement;

FIG. 10 shows a further embodiment wherein the handheld device issupported by a holding arm.

FIG. 11 shows a further embodiment wherein the handheld device issupported by a cable extending from a spring pulley.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The following description is focused on hip or knee surgery, inparticular treatment of femoro-acetabular impingement (FAI), uni kneearthroplasty (UKA) and total knee arthroplasty (TKA).

The part to be treated is a bone such as the pelvis, the femur and/orthe tibia, and the treatment consists in removing a planned volume ofthe bone.

However, the invention is not limited to these specific applications.

In particular, the part to be treated may not be a bone, and thetreatment applied to said part may not imply removing a volume of saidpart.

For example, the device can be used for drilling, sawing, milling bonesin any type of orthopaedic surgery performed to place various types ofimplants accurately (knee, hip, ankle, foot, wrist, shoulder, etc.) butalso in laser surgery or radiofrequency ablation of soft tissues, sawingand milling bones in cranio-facial surgery, reshaping teeth to fit withinlays or onlays in dentistry, drilling holes to place dental implants,inserting screws in bones for traumatology, drilling tunnels forligament reconstruction, performing one or multiple planar or domeosteotomies of bones, removing cement during a revision procedure,placing bone fragments accurately together, drilling inside the pediclesof vertebrae, removing cartilage defects, taking healthy parts of boneor cartilage in order to graft them, inserting inlays, implants, orgrafts at precise locations, placing needles accurately duringinterventional radiology procedures, etc.

As will be explained in further detail below, the device is used in acontext in which a volume of the part to be treated is planned beforethe surgical intervention.

Planning of the volume to be treated is performed using pre-operativeimages (e.g. CT, MRI, Ultrasound images, 3D X-rays, PET, etc.) orintra-operative 3D data (e.g. intra-operative CT, intra-operative MRI,Ultrasound images, 2D or 3D intra-operative X-ray images, geometric dataprovided by localizing systems and providing 3D points, clouds of 3Dpoints, surfaces reconstructed from clouds of 3D points, etc.)), orboth.

Multiple computer-assisted surgery methods are used to register theplanned volume with a coordinate system attached to the part to betreated.

Typically, intra-operative images or data are used to registerpre-operative images in a unique coordinate system attached to the partto be treated, and usually represented by a tracker (optical, magnetic,etc.).

Using any of these conventional computer-assisted surgery methods, theresult is that the volume to be treated has a known geometricrepresentation in a coordinate system attached to the part to betreated, and whose movements are tracked in real-time by a tracking unitas it will be detailed below.

As stated above, this volume may be a 1D, 2D or 3D volume depending onthe application.

General Description of the Surgical System

FIG. 1 shows an overview of a surgical system according to theinvention.

A patient P is lying on an operating table 500, e.g. in view of uni kneearthroplasty (UKA).

To that end, a tool 3 which is intended to remove a 3D volume from thetibial and femoral bones is supported by a handheld device 100 that ismanipulated by a surgeon (not shown).

The handheld device 100 is connected to a control unit 300.

Said control unit typically comprises power supply, AC/DC converters,motion controllers to power the AC/DC motors of the actuation unit,fuses, real time control system interface circuits.

The system also comprises a tracking unit 200, such that the pose of thedevice and/or the bone to be treated is tracked in real-time and isshared between a real time control system and a planning system.

At least one coordinate system 201 is attached to the part to be treatedwhile at least one coordinate system 202 is attached to the tool and/orthe handheld device.

The tracking unit measures the relative motions between both coordinatesystems 201, 202 at high frequencies.

The data obtained by the tracking unit are transferred to the controlunit 300 via any suitable connection, with wires or wireless (not shownhere).

The real time control system is able to carry out the proposed real timecontrol algorithms at a reasonably high frequency.

Based on the volume to be removed and the previously removed volume andthe actual pose of the device with respect to the bone to be treated,the real time control system calculates an optimal tool path.

There are many well-known algorithms in the robotics and machineliterature for optimal milling path generation based on geometricinformation like binarisation of the volume to be removed, oriso-parametric path generation algorithms from numerical controlmachining.

In this figure, the connection is represented by a wire 301 but it mayinstead be wireless if the handheld device is battery-powered.

The control unit and tracking unit may be arranged in a cart 302 thatcan be moved in the operating room.

The system also comprises a user interface 400 that is intended todisplay feedback information to the surgeon and enables systemconfiguration by the surgeon.

Said user interface 400 may advantageously comprise a screen located onthe same cart 302 as the control unit and tracking unit.

In addition to or instead of said screen, the user interface maycomprise an indicator that is arranged on the handheld device itself toprovide information to the surgeon.

A surgical system wherein the control unit, tracking unit and/or userinterface are embedded in the handheld device itself would still bewithin the scope of the invention, provided that the embedded units arepowered by a sufficiently powerful battery and that their size andweight do not hinder the manipulation of the device by the user.

Various embodiments of the handheld device will be described in moredetail below.

The handheld device (which may also be called “robot” in the followingdescription) comprises the following components:

-   -   a base, which is intended to be hand-held by the surgeon,    -   an end-effector on which a surgical tool dedicated to the        intended treatment can be mounted,    -   an actuation unit connecting the end-effector to the base, in        order to move the tool with respect to the handheld base for        treating the planned volume, said actuation unit having a given        number of degrees of freedom depending on the application.

Optionally, the handheld device may comprise a support unit connected tothe base or to the end effector and that comprises at least one elementintended to make contact with the part to be treated or an area of thepatient's body adjacent to the part to be treated so as to provide apartial mechanical link between the base or the end-effector and thepart to be treated.

Optionally, in combination or not with the above-mentioned support unit,the handling of the handheld device may be assisted by a holding armthat supports the base of the handheld device and that is connected to amechanical support.

Operation of the Surgical System

Before the surgical intervention, the user plans the intervention on theplanning system, based on pre-operative and/or intra-operative medicalimages and data.

This planning step is conventional and will not be described in detailhere.

It is specific to each application.

For example, in the case of TKA, planning of knee prosthesis on a femurrequires definition of five cutting planes on the bone. In the case ofFAI, planning of the volume to be removed on the femoral head and neckrequires definition of a complex shape to be removed in order toretrieve a shape that has normal geometric parameters, such as thesphericity of the femoral head.

The planning system may form part of the surgical system according tothe invention; otherwise, the planning system may be provided separatelyand connected to the control unit.

During the surgical intervention, the user may either use preoperativedata/images together with intra-operative registration methods, or usedirectly intraoperative data/images.

In both cases, the result of the planning consists of at least onecontinuous volume to be treated by the tool, the pose of said volume tobe treated being determined in the coordinate system of the part to betreated.

Said volume is then transferred to the control unit.

The control unit initializes its sub-systems and the device is ready touse.

If the robot comprises a support unit, before the treatment starts, thesupport unit has to be connected to the part to be treated or anadjacent body part to provide a partial mechanical link between thedevice and the part to be treated.

However, the invention may be carried out without any support unit andin such case the user handles the robot in his hand with a free motionanywhere in the space surrounding the patient's body. Anotherpossibility is that the user is assisted by the above-mentioned holdingarm that supports the robot.

Once the treatment has been started by the user, the control unitcontinuously feeds back status and tracking information to the planningsystem for recalculation and visualization purposes.

FIG. 2A illustrates an example of planning details that may be displayedby the screen.

Part p1 shows the already treated volume whereas part p2 shows thevolume that remains to be treated. The planned volume is the union ofthe already treated volume and the volume that remains to be treated.

In the following, it is considered that the surgical tool is a burr andthe part to be removed is an insert that corresponds to the negativeshape of an implant to be inserted in a bone. But this is only anexample and the invention is applicable to any other type of surgicaltool and part to be removed.

The global surface of the bone is referred to as S.

The dotted line p3 indicates the working space of the device, i.e. thespace wherein the tool can be used to operate safely.

Since the robot is hand held or mounted on an arm, it is compact and ithas a small working space. Having a compact device is advantageous incomparison with a cumbersome large robot because it takes less space inthe operating room. With a compact robot the working space is typicallyin the same order of magnitude as the volume of the part to be treated,which can be twice larger, or five times smaller as the volume of thepart to be treated for example. Having a small working space has theadvantage to offer a good level of safety since in the worst casescenario the robot can move only inside a very limited range. However,having a small working space is also a drawback in comparison with largerobots because it requires more efforts for the user to position therobot base at an appropriate location that will make it possible totreat the part in one step or in a limited number of steps. In thepresent invention, this drawback is compensated by innovative graphicaluser interfaces.

To inform the user which part of the volume to be treated can beactually treated from the current robot pose, a graphical representationof the working space of the robot and the part that remains to betreated is proposed.

The robot working space is defined by the union of all positions of thesurgical tool tip when all motors of the robot are activated from theirminimal value to their maximal value. This working space is definedrelatively to the base of the robot and therefore it can bepre-calculated off-line and then stored in a memory of the control unit.Geometrically, the working space is a volume that can be defined as aset of voxels for example (small cubes of a few tenth of millimeters) orby its boundary surface that can be represented by a mesh of triangles.

FIG. 2B illustrates the working space W for a robot 100 having a compactdesign with three motorized degrees of freedom and a spherical tool tip(burr 3). The actual robot working space can then be calculated in thesame referential as the volume to be treated using a tracking unit witha sensor attached to the base of the robot and a sensor attached to thepart to be treated. If the base 1 of the robot is not equipped with asensor to be tracked, a tracking sensor is attached to the end-effectoror to the surgical tool and it is measured in real-time, then the poseof the base is estimated from the values of the intrinsic encoders ofthe robot and the direct geometric model of the robot that defines therelation between its base and its end-effector or surgical tool as it iscommonly done in robotics. In the following, all representations aretherefore calculated in the referential of the part to be treated.Alternatively, the robot base could be used as a reference.

Different representations of the working space of the robot and the partto be treated are proposed and can be implemented using softwarepackages such as OpenGL or Open Inventor. In general, three dimensionalrepresentations projected on a two-dimensional view are preferred andcan be implemented using standard display monitors. However, it is alsopossible to use stereoscopic or holographic modes of visualization toreinforce the three dimensional characteristics. Alternatively, symbolictow-dimensional representations can also be used.

In a preferred embodiment, a three dimensional view of the volume thatremains to be treated p2 is displayed together with the volume alreadytreated p1, both are encapsulated in the global shape of the bone orbody part.

FIG. 2C illustrates such representation of volumes p1 and p2. Inpractice, the external surfaces of volumes p1 and p2 are appearing tothe user. The external surfaces of p1 and p2 can be differentiated bycolors, textures, etc. or they can be left without differentiation. Inthe latter case, a color map can be used to indicate if milling isnecessary to be performed in all areas, for example green indicates thatthis area has been milled to its desired objective and red indicatesthat this area has not been milled at all, with orange and yellowindicating intermediate situations. In that situation, the volume p2will appear directly at the area in red, orange, yellow whilst the areap1 will appear directly in green.

In a preferred embodiment, the working space p3 is overlaid as atransparent object, with a specific and distinct color. It isrepresented on FIG. 2D. When the user moves the robot base, he sees theoverlay p3 moving accordingly on the three dimensional view of p1 and p2and he can adjust the base position to cover the maximum part of p2. Ifthe robot working space is sufficient, the user will attempt to find aposition that covers p2 totally or to a very large extent. Suchembodiment has the advantage to make the global process fast and optimalin time, and the task of the user is only to position the base and towatch carefully the milling process (and stop the milling process if therobot is moving inadvertently). In the contrary, the user will select aposition of the base that will produce a situation wherein p3 covers asubstantial part of p2, then the user will activate the milling processfor that area, then he will move the base to reach an adjacent, and williterate this process until all remaining areas p2 have been removed.Such embodiment has the advantage to increase the safety of the devicebecause only very small parts of the patient body can be removed for agiven position of the robot base, and the user has an increased controlover the device.

By activating a switch, the representation of the working space willdisappear and the user will see a standard representation of a millingprocess with typically a color map from green to red indicating thedepth that remain to be milled, for example red means that maximum depthremain to be milled and green means that the area has been milled to thetarget, orange and yellow being intermediate indicators. In a preferredembodiment, said switch is the same switch that is used to activate themilling process. When the switch is released, the graphical displayautomatically comes back to representations of the working space, withthe possibility to switch again to the conventional view without theworking space.

In another preferred embodiment, the working space of the robot isrepresented by a mesh of lines instead of a transparent object. Theportions of the lines that are behind the surfaces of p1, p2 and thebone surface S are hidden and it renders a good impression of therelative depth of the working space with respect to the surfaces of p1,p2 and S. It is represented on FIG. 2E.

In another preferred embodiment, for a given position of the base and acorresponding working space of the robot, the intersection between thevolume of the working space and the global surface of the bone S fromwhich the volume p1 has been removed and which is denoted S′ iscalculated, it generates a surface patch SP. The surface patch SP isoverlaid on the volumes p1 and p2 as well as the global surface S′. Thesurface patch SP can be represented by a mesh of triangles, with atransparent color, and/or reinforced edges on its boundaries. It isillustrated on FIG. 2F. It has the advantage to offer a surfacerepresentation instead of a volume representation, and it is thereforeeasier to interpret by the user. However, it might be less accurate.

In order to compensate for inaccuracies issues as well as small motionsof the base of the robot during the milling process, in a preferredembodiment, the real working space of the robot is reduced to a safeworking volume. This reduction can be calculated in a Cartesian spacedirectly on the volume of the nominal working space or it can becalculated by reducing the amplitudes of the minimum and maximumpositions of each joint of the robot. The amount of reduction of theworking space can be an adjustable value that can be fixed for a givenapplication and for given environment (use expertise, patientimmobilization using leg holders, etc.).

In another preferred embodiment, the surface patch SP is truncated tothe volume of p2. It is illustrated on FIG. 2G.

In another embodiment the representation of the working space and thearea to be treated is more symbolic and limited to a two dimensionalview. The following task is considered as an example, as illustrated onFIG. 2H. The robot 100 has three degrees of freedom and the surgicaltool 3 is a spherical burr, and the task is to mill a cylinder shape Cin a bone, such a cylinder will host the peg of an implant. The userneeds to position the base 1 of the robot in order to align thetranslation axis of the robot with the axis of the cylinder with someamount of tolerances which depends on the diameters of the cylinder, thespherical burr and the rod that carries the spherical burr. If thealignment is not reached within such tolerances the task will not beable to be performed totally and the user will have to reposition thebase of the robot. FIG. 2I shows a possible representation intended tohelp the user positioning the robot base directly on an appropriatepose. The outer circle c1 represents the entry of the cylinder and theinner circle c2 represents the bottom of the cylinder in a perspectiveview aligned with the cylinder axis. The current position of the burr 3is overlaid. The working space p3 of the robot is calculated in threedimensions and then projected on the same perspective view, and itappears as a complex shape around the circles. The user simply needs toensure that the complex shape encloses the outer circle. This situationis represented on FIG. 2J. When this situation has been reached, theuser can lock the base position and launch the milling process. If thebone does not move much relatively to the robot base, the milling of thecylinder will be reached in a single step. As previously described, theworking space can be reduced to offer a safety margin.

In a preferred embodiment, it is also possible to compute and displaycriteria that indicate a level of efficiency to the user. For example, auseful criteria is the percentage of the volume p2 that will bereachable for a given position of the base. Indicating one hundredpercent would represent the highest score possible.

Returning to FIG. 2A, part p4 is the tool pose with respect to the boneto be treated as measured by the tracking unit and/or calculated by thereal time control unit.

During use of the handheld device, the user is provided with informationregarding repositioning of the device to be carried out in order tomaintain the tool in a predetermined working space.

According to the invention, the user benefits, on the one hand, from afree manipulation of base of the handheld device and, on the other hand,from the control unit's automatic tool path optimization.

When a support unit is used, the partial mechanical link provided by thesupport unit enables the surgeon to make small movements to repositionthe device, without additional invasive action on the patient.

In addition, the user interface provides information to the user aboutthe ability to treat the planned zone in the current device positionand, if appropriate, gives indications on how to reposition the deviceappropriately.

FIG. 2K provides an example of a two-dimensional representation of therobot and the planned volume, along with an indication (here, an arrow)regarding how the base should be repositioned.

Such two-dimensional representation could be displayed in e.g. threedifferent views.

This embodiment is particularly advantageous when the robot includes asupport unit 5. Indeed, the indication for the repositioning of the basetakes the constraints of the support unit into account. In such case,the user just has to reposition the base 1 without changing the support(e.g. by pivoting around the tip of the support unit 5), which is mucheasier as compared to a situation where the repositioning of the baseinvolves six degrees of freedom.

The system ensures intervention safety as the control unit stops thetool if it leaves the planned working zone. In addition, the device'sworking space is small, providing inherent safety.

Another advantage of small working spaces is higher accuracy due tosmaller leverages.

General Description of the Handheld Device

FIGS. 3A to 3C illustrate examples of embodiments of the handhelddevice.

FIGS. 3A to 3C are intended to provide a general description of thehandheld device, although the invention may be implemented according tovarious embodiments.

A description of some specific embodiments—related to specific surgicalapplications—will be described below.

Base

The handheld device comprises a base 1, which is intended to behand-held by the surgeon.

To this end, a specific handle may be mounted on the base, or the basemay itself be designed so as to provide an ergonomic shape.

Tool/End-Effector

The handheld device further comprises an end-effector 2 on which asurgical tool 3 dedicated to the intended treatment can be mounted (seeFIGS. 3A-3B).

According to an embodiment, the tool 3 may be factory-mounted on theend-effector 2; otherwise, the end-effector may comprise an attachmentsystem (e.g. a clip-on mechanism) to secure a tool which may be aconventional tool provided separately. This embodiment has the advantageof being easily compatible with a sterile drape used to cover theactuation unit and the end-effector.

As a result, the tool 3 may or may not be part of the device.

According to another embodiment (see FIG. 3C), the end-effector supportsa guide 20 for the tool dedicated to the intended treatment.

When the tool is a saw, the tool guide is a cutting guide that defines acutting plane (2D volume to be removed).

When the tool is a burr, the tool guide is a drilling guide that definesa milling line (1D volume to be removed).

The tool guide 20 may be factory-mounted on the end-effector 2;otherwise, the end-effector may comprise an attachment system (e.g. aclip-in mechanism) to secure a conventional guide.

The tool guide may be fixed or movable (e.g. slidable and/or rotatable)with respect to the end-effector.

Actuation Unit

As shown on FIGS. 3A-3C, the end-effector 2 is connected to the base 1by an actuation unit 4, in order to move the tool 3 or, if appropriate,the tool guide 20, with respect to the handheld base for treating theplanned volume.

The actuation unit has a given number of degrees of freedom depending onthe application.

The actuation unit 4 comprises motors, gears (optional) and sensorsconnected together to form a kinematic structure.

As it will be explained in more detail below, the actuation unit 4 iscontrolled by the control unit 300.

According to one embodiment, the tool 3 can be a spherical burr orshaver and the volume to be removed is modeled as the union of sphereshaving the same diameter as the burr or shaver.

In such case, the actuation unit 4 is designed so as to have threedegrees of freedom (in addition to one degree of freedom to activate therotation of the burr).

A further optimization may comprise five degrees of freedom to adjustthe orientation of the tool's cutting edges relative to the part to betreated.

In another embodiment, the tool can be a cylindrical burr or shaver andthe volume to be removed is modeled as one possible union of cylindricalsegments having the same shape as the burr or shaver.

In such case, the actuation unit is designed so as to have five degreesof freedom.

The volume to be removed can also be represented by voxels, triangulatedmesh data, coordinates of a plane (for sawing mode); points, directionsand depth (for bore holes). And the actuation unit design will beadapted to the type of volume to be treated.

It is possible to make the actuation unit a sterile component, to besterilized before each intervention. But, in a preferred embodiment, theactuation unit and its cables are covered by a single-use transparentplastic sterile drape. Additional components of the system can be alsoprotected under the sterile drape. But the tool itself is sterile, likeany conventional tool. Typically, it is sterilized before eachintervention using autoclave. Different types of mechanical adaptorsbetween the sterile drape and the tool can be provided. Such adaptordoes not require a very precise reproducible fixation if the toolcontains a tracking element, which facilitates the design and the use ofthe global system.

Support Unit (Optional)

Optionally, the handheld device may further comprise a support unit 5which is connected either to the base 1 (see FIGS. 3A-3B) or to theend-effector 2 (see FIG. 3C).

Said support unit 5 comprises at least one element intended to makecontact with the part to be treated or an area of the patient's bodyadjacent to the part to be treated so as to provide a partial mechanicallink between the base 1 (or the end-effector 2 as shown in FIG. 3C) andthe part to be treated. The user is trained to always apply pressure tothe support unit to ensure sufficient contact with the part to betreated.

The support unit 5 is usually a sterile component. The connectionbetween the support function and the base or end-effector can beestablished on the sterile drape if the actuation unit with its base andend-effector are covered with a sterile drape. Or it is establisheddirectly if the base or end-effector is sterile.

When used, the support unit 5 acts as a stabilizer of the handhelddevice.

Said support unit may be rigid, damped (e.g. spring-loaded) and/orprovide adjustable damping properties.

The contact between the support unit 5 and the body part may be made ofone or several points or of at least one surface.

In the embodiment shown in FIGS. 3A and 3C, the support unit 5 comprisesa rigid pin in contact with the part P1 to be treated.

According to an alternative embodiment (see FIG. 3B), the support unit 5comprises a damped pin which is in contact with a part P2 of thepatient's body adjacent to the part P1 to be treated.

According to an embodiment, when the end-effector 2 supports a toolguide 20, the support unit 5 may comprise the tool guide 20 itself (seeFIG. 3C).

In each case, due to the fact that the support unit 5 makes eitherdirect contact with the part P1 to be treated itself or indirect contactvia a region P2 of the patient's body adjacent to part P1, the supportunit 5 has the effect of a partial mechanical link that limits themovements of the user when operating the device—and, in advantageousembodiments, also damps movements of the user, vibrations of the cuttingtool and reaction forces caused by movements of the actuation unit.

In addition, as the support unit only provides a partial mechanicallink, it does not require any invasive action.

Without this limitation effect, all user movements with respect to thepart to be treated would have to be compensated in real time by theactuation unit, which is extremely difficult to achieve with existingrobotic technologies.

Micro or macro motions of the user with respect to the part to betreated, including slow and fast motions, must be compensated within atolerance range that defines the device's precision.

Typically, for bone surgery applications, motions in the range of a fewtenths of a millimeter need to be compensated to obtain sufficientprecision; such compensation requires ultrafast motion detection andmeasurement, as well as calculation of the compensation motion to beapplied and execution of desired compensation motion. Non-compensatedmotions result in bumpy or irregular surfaces.

For dental applications, the required precision is even higher,typically in the range of a few hundredths of a millimeter.

Sensors, computers, motors, controllers, low inertia mechanisms able toachieve the performance described above do not exist or would beextremely costly to develop and manufacture.

By considerably limiting—and preferably damping—motions, the supportunit makes it possible to use existing robotic technologies to obtainthe required precision.

The partial mechanical link is also important when the part to betreated is being treated. The support unit enables a short closed forceloop of the forces exerted during treatment via the device back to thepart where the support unit is on.

Only a small quantity of the forces exerted during treatment needs to becompensated by the user, the result of which is higher accuracy.

Depending on the type of support unit, parts of the forces exerted willbe damped by the closed force loop.

For example when using one sharp pin as support, the forces in twodirections can be damped parallel to the surface where the pin islocated.

For example, the partial mechanical link may be achieved by one or moresharp-tipped pins able to prick the bone when the device is pressed onthe bone by the surgeon.

Such a sharp pin is referenced to as 50 in FIG. 3D.

Such a sharp pin 50 creates a partial link consisting of three degreesof freedom.

When two sharp pins are used, only one degree of freedom is allowedabout the axis passing through both pin tips.

The partial mechanical link may also be achieved by one or more pinshaving a rounded tip which may be pressed by the user onto the bone whenusing the device (see reference 51 in FIG. 3D).

The tip of the pin may also have a silicone disc (not shown) attached toit providing a damping effect.

According to another embodiment, the support unit may comprise at leastone wheel provided with several sharp teeth, such a wheel being able toslide on the bone in at least one direction and optionally allowing arotation about the wheel axis (see reference 52 in FIG. 3D).

According to another embodiment, the support unit comprises anattachment device having a surface attached to or at least in contactwith a part of the patient's body so as to fit the shape of said partand at least one pin connecting said attachment device to the base orthe end effector of the handheld device. The attachment deviceadvantageously prevents the pin end from sliding with respect to thepatient's body. The connection between the pin and the attachment deviceis preferably reversible and can make use of any type of connection(mechanical, magnetic, etc.). For example, the connection may includehoop-and-loop fasteners such as Velcro®, magnetic fasteners (e.g. theattachment device comprising a metal and the pin end comprising amagnet), or a mechanical articulation.

As shown in FIG. 3E, the support unit 5 may also comprise a tourniquet53 to which the support pin end is connected and that acts as a strapcompressing the soft tissues around the bone, thus creating a non-rigidlink that is standard and well accepted for other purposes. The termtourniquet is used for defining a strap attached around the soft tissues(skin) with or without the function of stopping blood. The connectionbetween pin end and tourniquet bears shear forces and thus prevents thepin from sliding on the soft tissue. As mentioned above, the connectionmechanism may be designed as hoop-and-loop fastener or a magneticfastener.

An alternative embodiment may comprise an adhesive patch 55 instead ofthe tourniquet, which is attached to the skin of the patient andconnected to the pin end (FIG. 3F). The external surface of the patchadvantageously prevents sliding of the pin end.

Alternatively, the attachment device may be formed by applying amoldable material onto a part of the patient's body. For example, pastesused in dentistry are able to be molded and to harden quickly, possiblyassisted by a heater without any damage to the patient, by using athermoforming principle. The molded device thus forms a template thatfits the shape of the patient's body.

According to another embodiment, illustrated in FIG. 3G, the supportunit 5 may comprise a pin 54 a, one end of which is connected to thebase 1 and the other end of which is connected to a deformable cushion54 b (similar to a sand sack), which provides a damped support function.

Such a cushion is intended to be placed on the patient's skin, in avicinity of the entry port of the tool.

Optionally, such a deformable cushion 54 b may be fixed by vacuum inorder to provide more rigid support.

In a similar embodiment, illustrated in FIG. 9A/9B, the end of the pin54 a may be disk-shaped 54 c, made of plastic for example, with asurface intended to be in contact with the patient's body. Such a pinend may be coated with a flexible type of material (such as silicone orfoam) for better adhesion to the patient's skin.

The pin is generally made of metal, e.g. steel.

The connection between the pin 54 a and the cushion 54 b or the disk 54c may take the form of a ball and socket joint (not shown).

The ball is advantageously at the end of the pin 54 a and is made ofmetal, whereas the socket is integrated into the cushion 54 b or thedisk 54 c and is made of plastic.

The ball and socket joint can thus be easily connected and disconnectedby elastic deformation of the socket.

Consequently, the user may first install the cushion 54 b or the disk 54c in a suitable place on the patient's body, then take the handhelddevice 100 and connect the pin 54 a to the cushion 54 b or the disk 54 cvia the ball and socket joint to assemble the support unit 5.

Conversely, when the user stops using the handheld device 100, the usercan disconnect the pin 54 a from the cushion 54 b or the disk 54 c andlater move or remove the cushion 54 b or the disk 54 c from thepatient's body.

The support unit 5 may advantageously provide damping such that saidpartial mechanical link created between the base and the part to betreated is able to absorb reaction forces exerted by the treated partonto the tool.

Said damping characteristics may be provided by a spring-loaded elementor by a pneumatic or hydraulic damper.

Advantageously, the damping characteristics of the support unit may beadjustable.

Damping presents the advantage of higher accuracy when the user does nothave to compensate the reaction forces.

In another embodiment, illustrated in FIG. 3I, the support unitcomprises at least one pin 56 having a V shaped end with circular crosssections, providing one not stabilized degree of freedom between baseand patient.

This degree of freedom may be blocked by using two of such these pins 56(FIG. 3H) or by using a pin 57 having an end formed like an anglebracket (FIG. 3J). The V shape of the pin end(s) provides a template foralignment with the tibial crest line and thus a possibility for coarsepositioning of support unit and handheld device.

This embodiment is particularly advantageous in UKA because the supportunit provides a limited number of positions that make it possible toreach both areas to be milled on the tibia and on the femur, in a quickan intuitive manner.

More generally, the support unit can be designed to have a predefinedshape that fits the external surface of a patient anatomy such that therobot base will be positioned immediately in an area wherein thecorresponding robot workspace will match the volume of the part to betreated.

Optionally, as shown in FIG. 3H, a combination with the above-mentionedtourniquet 53 or adhesive patch 55 with Velcro® or magnetic fasteningmechanism is possible in order to prevent sliding of the V shaped pinend(s) with respect to the tibial crest.

According to an embodiment illustrated in FIG. 3K, the support unit 5comprises an attachment device 58 such as a tourniquet, an adhesivepatch or a template as described above, attached to a part of thepatient's body (here, P2 designates the tibial crest). A mountingelement 59 is fixed to the attachment device 58. The base of thehandheld device is coupled to the attachment device by a connectingmember in sliding engagement with the mounting element 59 (translationmovement T along the tibial crest). The connecting member advantageouslycomprises a joint that allows rotation R1 of the base. If the endeffector or the tool is also allowed to pivot with respect to theactuation unit 4 (rotation R2), the tool is able to reach all or almostall necessary different parts of the patient's body without moving thebase of the handheld device.

According to an embodiment illustrated in FIG. 3L, the support unit 5may be designed as a second hand grip for the user (in addition to thepart of the base that is normally held by the user). For example, inFIG. 3K, this hand grip is designed as a ball that can be held in thesecond user's hand, the first hand holding the base. Then the partialmechanical link between the patient and the device is provided by thesecond hand of the user that leans on the patient's body, which is ableto stabilize and damp relative movements.

According to an embodiment (not illustrated), the support unit may alsocomprise at least one sensor for detecting a force exerted by the useronto the partial mechanical link; hence, the control unit is able tocheck whether said force is greater than a threshold such that thesupport unit has a minimal damping parameter. This ensures that forexample the sharp pins may not slip on the bone.

If not, the control unit may emit an alert to the user or automaticallystop the device.

Depending on the application, the above-described embodiments of thesupport unit may be combined to provide a suitable partial mechanicallink between the handheld device and the patient's body.

The support unit 5 is preferably designed so as to be disconnectablefrom the handheld device 100, in particular for sterilization and/orreplacement of the element providing the support function.

For example, but non-limitatively, ball and socket joints may be used toconnect and disconnect the support unit to the base or to theend-effector easily.

If the support unit 5 is connected to the base 1, it only generates aglobal constraint that will prevent the user from placing the tool 3anywhere in a large space, but it will not generate any limitation toreach any volume or target in its vicinity.

The actuation unit 4 will continue to act and compensate for therelative motions between the base 1 and the part 100 to be treated,whilst continuously optimizing the milling path.

The support unit 5 is intended to help solve the complex control anddynamics of the system but not to add a geometric constraint in a localneighborhood.

At some point, the support unit 5 may generate a global constraint thatrequires the user to move the base and/or the support unit contactareas.

Tracking Unit

The system also comprises a tracking unit 200 configured to determine inreal time the pose of at least one of the tool 3, the end-effector 2 andthe base 1 with respect to the part 100 to be treated.

The pose of the tool 3 relative to the end-effector 2 may be eitherknown and mechanically determined, or not.

Knowledge of the tool pose relative to the end-effector may be useful inview of redundancy calculation based on kinematics.

The tracking unit may typically comprise a tracking system, which isknown per se.

Tracking systems commonly used in computer-assisted surgery use avariety of different technologies (passive optical, active optical,electromagnetic, inertia with gyroscopic measurements, ultrasonic, etc.)that can be used individually or in combination.

The tracking unit measures the pose of a first object equipped with atracker with respect to another reference second object also equippedwith a tracker.

In some cases, the reference second object is the tracking unit itselfand only one tracker is necessary on the first object.

A tracker can be an emitter or a sensor (also called receiver).

In all cases, the tracking unit provides a transform matrix between thepose of the part to be treated and the pose of the tool and/or thedevice.

The typical frequency of the tracking unit is in the range of 50 Hz to1000 Hz or even more.

In the appended drawings, emitters 210 are represented by a trianglewhereas sensors 220 are represented by a square.

Emitters and sensors can be redundant to increase accuracy of posemeasures.

In the example of an electromagnetic tracking system with severalpossible configurations for emitters and sensors, we consider that themeasure obtained between an emitter attached to a first object and asensor attached to a second object fully determines the relative pose ofthe first object with respect to the second object. Said pose is usuallyrepresented by a matrix representing a 3D rotation and a vectorrepresenting a 3D translation.

If a sensor is attached to a third object, elementary combination ofmatrices determines the relative pose of the third object with respectto the second object (that is to say between two sensors).

As shown in FIGS. 4A-4E, various arrangements of emitter(s) andsensor(s) are possible.

Preferably, the distance between the sensor and the emitter is minimizedso as to minimize the possibility of distortions induced by surroundingmetallic objects.

According to a preferred embodiment, the emitter is mounted on thedevice itself, more precisely on the base or on the end-effector.

Only one sensor mounted on the part to be treated may be sufficient.

This has the advantage of reducing the number of necessary coordinatesystem transformations and thereby reducing error occurrence andincreasing accuracy.

In some cases, e.g. when the tool is a shaver or a small diameter burr,not rigid enough to allow tool tip pose determination based on the poseof the end-effector, an additional sensor may be mounted on (or closeto) the tool tip.

In FIG. 4A, the tracking unit comprises an emitter 210 located on theend-effector 2 and a sensor 220 located on the patient's body (here, ona part P2 adjacent to the part P1 to be treated).

In FIG. 4B, the tracking unit comprises an emitter 210 located on thetool 3 and a sensor 220 located on the patient's body (on a part P2adjacent to the part P1 to be treated).

In this particular embodiment, the support unit 5 and the tool guide 20are connected to the end-effector 2 and are spring-loaded, so that undera pushing action the tool 3 (here, a sawblade) can slip out of the guide20.

In FIG. 4C, the tracking unit comprises a sensor 220 located on the tooltip and the emitter 210 on the part P1 to be treated.

In FIG. 4D, the tracking unit comprises an emitter 210 located in theoperating room and two sensors 220: one on the end-effector 2 and theother one on the part P1 to be treated.

In FIG. 4E, the tracking unit comprises an emitter 210 located on thebase 1 of the device and a sensor 220 located on the patient's body.

In addition, it should be noted that knowledge of the actuation unitkinematics and end-effector geometry may also be used to determine thetransform matrix between the base pose and the tool pose. If an emitteror sensor is attached to the base, and a sensor is attached to the tool,one obtains redundant measurements that are used for safety andoptimization purposes.

Of course, the above-described embodiments are only examples and askilled person may arrange the sensor(s) and emitter(s) in a differentway, or combine some of these embodiments, without departing from thescope of the invention.

User Interface

As mentioned previously, a user interface is defined so as to show theuser a safe device position.

In most cases, intrinsic safety is achieved when the working space ofthe tool is smaller than the volume to be treated. This also results insmall device dimensions and thus a lightweight and compact handhelddevice.

The user interface may provide information to the user to guide him orher to reposition the device continuously in an optimal pose.

Said user interface may be haptic, visual and/or acoustic.

According to an embodiment, the user interface 400 may comprise a screen410 connected to the control unit, e.g. the screen shown on FIG. 1.

As shown in FIG. 5A, said screen 410 displays, e.g. in the form of anarrow, the adjustments necessary to maintain continuous treatment of thewhole volume.

This adjustment information may also be visualized on an endoscopicimage (e.g. in FAI treatment).

During the use of the device the control system checks in real time ifthe volume to be treated can be processed safely. If the robot base ismoved such that the border of the workspace comes closer to the volumeto be treated, meaning that there is not much space for errorcompensation, then the information provided to the user may change, e.g.the arrow changes its color or an acoustical feedback is produced.

According to another embodiment (see FIG. 5B), the user interfacecomprises optical indicators 420 such as LEDs supported by a supportingplate 421 that is fixed to the device 100.

Said indicators 420 are connected to the control unit 300 and placed soas to show the user in which direction and/or orientation the device hasto be moved to ensure non-interrupting treatment.

The LEDs can be multi-colored or blink at different frequencies toindicate how much and/or how soon correction in the pointed direction isnecessary.

Since the indicators 420 are arranged on the device 100 itself, thesurgeon receives this pose information in situ, which is very convenientfor him or her.

Another way of providing information to the user is an opticalprojection of repositioning information directly on the patient's skinor bone.

For example, as shown on FIG. 5C, the information may take the form ofan arrow 430 projected by a laser pointer (not shown) arranged on thedevice 100 and controlled by the control unit 300.

Control Unit

The system further comprises a control unit which is intended to controlthe tool path in an optimal way in order to treat the planned volume.

Operation of the control unit will be described in more detail below.

FIG. 6 is a synoptic drawing showing a possible operation of the system.The method described here is suitable for UKA and treatment of FAI.However, a skilled person will be able to adapt it for other surgicalprocedures.

The planning phase is carried out before system operation.

In step S100, the user starts operating the device described above.

In step S101, the pose of the tool tip with respect to the part to betreated is measured by the tracking unit.

In step S102, the control unit updates the remaining volume of the partto be treated which at the beginning of the process is the entireplanned volume to be removed.

In step S103, the control unit determines the pose of the end-effectorwith respect to the base.

Based on the planned volume to be treated (input 1104), the control unitdetermines the remaining volume to be treated (step S105) and, based onthe pose of the end-effector with respect to the base, determines thepart of said volume that can be treated safely (step S106).

Safely means here, that at any position of the safely removable volume,the device is able to compensate for errors caused by movements inducedby the user. This could be that the device is able to move the tool fora certain defined amount of millimeters with a determined speed orwithin a determined time or rotate it by a certain defined amount ofdegrees. To be able to do this, the dynamic behavior of the end-effectorat each pose with the working volume has to be known.

In step C107, the control unit checks whether the tool tip is inside theplanned volume or not.

If not, the control unit stops the tool and device immediately andgenerates a message for the user (step S109).

Advantageously, the position data may be previously filtered so as toavoid stopping the device if outlier values are obtained.

If the user acknowledges this alert (step C110) then the method beginsagain from step 101.

To that end, the user has to reposition the device as may be indicatedby the user interface (preferred embodiments of such a user interfaceare described above).

If the tool tip is inside the planned volume, the control unit checkswhether the safely treatable part of the volume is above a giventhreshold (step C108).

The threshold may be implemented in different ways. One could be thatthe size of the remaining volume is smaller than a defined size. Anotherimplementation could be that the distance to the border of the safelyremovable volume is smaller than a defined value.

If not, the control unit 300 indicates, via the user interface 400, thatthe user should reposition the device in a calculated direction (step115). This repositioning may include changing the position of thesupport unit, or may be made without changing the position of thesupport unit (e.g. by moving the base along the free degrees of freedomof the support unit). For example, the user interface may indicate topivot the base of the device around a fixed support tip, in a givendirection and for a given amount, until it has reached a pose that willallow reaching a new sub-volume of the part to be treated, saidsub-volume having a size above a predefined threshold. Then the methoditerates again from step 101.

If the safely treatable part of the volume is above said giventhreshold, the control unit calculates an optimized path of the tool tipin the coordinate system of the device (step 111). The optimized pathmay comprise not only poses at specific time steps but may also includetool parameters, such as changing the rotation speed of the millingdevice depending on the tool pose. In other words, the optimized pathmay contain poses according to six degrees of freedom and processingparameters at each time step.

If the user enables the process via hand or foot switch (step C111 b),then the control unit adjusts the actuation unit so as to move theend-effector according to said computed path (step 112). As the path isdefined in coordinates of the sensor on the part to be treated, and thecontrol unit minimizes the distance between the tool tip and the currentpose of the computed path at each time step, the device will follow thepath, independently of movement that the user applies to the device.

The control unit checks whether the remaining volume to be treated isbelow a given threshold (step C113).

Said given threshold may be defined by the user as a limit equating to asatisfactory treated volume, even if the treated volume is not exactlyidentical to the planned volume.

In the affirmative, the method ends (step S114).

If not, the method iterates again from step S101.

The iteration may be carried out at high frequency (e.g. 1000 kHz).

In the case of TKA, the method differs slightly from the methoddescribed above.

For this intervention, there are two possibilities:

-   -   first, the tool (a sawblade) is distinct from the device and is        handled in a first hand of the user (or an assistant) whereas        the device is handled in a second hand (of the user or of an        assistant) (see FIG. 4E for example),    -   second, the tool is part of the device and, by pushing the        device onto the bone, the tool slides out of the guide into the        bone (as shown in FIG. 4B).

In step S101, the pose of the end-effector or the support unit withrespect to the bone is determined.

In step C113, it is the user who decides whether the planned 2D volumehas been removed.

Depending on the application, it is possible that the base requiresre-positioning several times, typically three or four, but sometimes asmany as one hundred. However, when a large number of re-positioningactions are necessary, discrete positions of the base are notnecessarily required. Instead the base may move continuously and slowlyto cover the necessary complete working space. For complex shapes to betreated, the user may be asked to move the base continuously for a givenposition of the support unit, then move the support unit to anotherfixed location, and then move the base continuously in that vicinity,and to repeat this process until the complete volume has been treated.

The repositioning of the base or of the support unit may be guided bythe user interface so as to reach a new position from which without anew change maximal volume can be removed. This is to minimize the numberof necessary repositioning steps.

As mentioned above, the optimized path may comprise poses and processingparameters (e.g. the tool speed) at each time step.

Holding Arm

According to one embodiment, illustrated in FIG. 10, handling of thehandheld device 100 may be assisted by a holding arm 510 that supportsthe base 1 of the handheld device 100 and that is connected to amechanical support.

The holding arm 510 is articulated with several degrees of freedom and aswitch is used to brake or freeze its position (pneumatic arms,hydraulic arms, mechanical arms, arms with brakes, etc.). In addition, aholding arm may have a mechanism to compensate for the weight of thehandheld device that it carries (by using passive or activecounterweights for example).

In this case the holding arm is considered to be another possibleembodiment of the support unit previously described.

Another variation is to use a local support element in addition to theholding arm, similar to the embodiments of the support unit previouslydescribed.

The holding arm 510 is articulated so as to allow the user to positionthe handheld device 100 in a desired position.

The holding arm 510 allows compensation for the weight of the handhelddevice 100 and minimizes user tiredness, especially if the surgicalprocedure takes a long time.

Preferably, said mechanical support is in contact with the patient'sbody.

The mechanical support also contributes to create a partial mechanicallink between the part to be treated and the handheld device.

The mechanical support is preferably the operating table 500 or asupport element which is commonly used to position and hold the patient(e.g. leg holder, pelvis holder, operating table post, etc.).

Additional local support may be achieved by integrating a supportelement on the end-effector 2 or the base 1 as described above to have asmaller force chain to compensate, with fewer vibrations. Thisembodiment is particularly useful if the mechanical support of theholding arm is simply a mobile cart or is ceiling-mounted.

Holding Cable

According to a further embodiment, illustrated in FIG. 11, handling ofthe handheld device 100 may be assisted by a cable extending from aspring pulley 511 that supports the base 1 of the handheld device 100and that is connected to a mechanical support 511 a.

The holding cable 511 allows all degrees of freedom of the handhelddevice at all the time of the use.

In addition, the holding cable may have a mechanism to compensate forthe weight of the handheld device that it carries (by a weight adjustedfor the return spring to wing the cable, for example) in order tominimize user tiredness, especially if the surgical procedure takes along time. In that way, when the handheld device 100 is released, itremains in its position or back up slightly.

The mechanical support 511 a can be either supported by the ceiling 512of the room, by a fixed or mobile cart 513 resting on the floor, or bythe operating table.

A partial mechanical link may be provided by a support unit (not shown)on the end-effector 2 or the base 1 as described above.

Example of Application No. 1—Uni Knee Arthroplasty (UKA)

In UKA, the tool is used to prepare tibial and femoral bone for implantfixation at a planned position and orientation. The volume to be removedcoincides with the negative shape of the implant when it is in its finalposition.

The used milling strategy could also be used for all other applicationswhere a cavity has to be removed or the bone surface has to be reshapedas when using patient specific implants.

In this respect, a (3D) bone volume both at the femoral and at thetibial side has to be removed for the insertion of the implantcomponents.

To that end, the tools used are typically mills and shavers.

FIGS. 7A-7B illustrate a particular embodiment of a suitable handhelddevice 100.

Here, the tool 3 is a mill attached to the end-effector 2.

The actuation unit and the end-effector are decomposed into a frontalactuation unit 4 a and the corresponding end-effector 2 a, and a backactuation unit 4 b and the corresponding end-effector 2 b.

The handheld device 100 provides three degrees of freedom:

-   -   two degrees of freedom are provided by the frontal actuation        unit 4 a (here, the actuation unit comprises a planar five-bar        linkage),    -   one degree of freedom (feed motion off the pivot point) is        provided by the back actuation unit 4 b.

Optionally, the support unit 5 comprises one sharp pin 50 that isconnected to the base 1.

The sharp pin 50 is intended to be in contact with the bone adjacent tothe bone to be treated.

In use, the user pushes the support unit 5 onto the bone adjacent to thebone to be treated so as to create the partial mechanical link.

Example of Application No. 2—Total Knee Arthroplasty (TKA)

In TKA, the tool is used to prepare tibial and femoral bone for implantfixation at a planned position.

The tools generally used in such an intervention are saws and drills.

Typically five planar cuts at the femur and one planar cut at the tibiaare done.

According to a first embodiment, the tool is handheld by the user and atool guide is fixed to the end-effector of the handheld device.

In this way, the tool itself is freely manipulated by the user in onehand but it is guided by the tool guide whose pose is defined in asuitable manner by the system according to the invention, the handhelddevice being held in the other hand of the user or by an assistant.

FIGS. 8A-8B shows a particular embodiment of such a handheld device.

The tool guide 20, which is here a saw guide (but it could of course bea drill guide if the tool is a drill), is attached to the end-effector2.

According to an alternative embodiment, the tool 3 itself is fixed tothe end-effector.

A tool guide is not necessary in this case.

FIG. 8C shows a particular embodiment of such a handheld device.

In this case, the tool 3 is a saw that is arranged at the end of theend-effector.

The handheld device provides six degrees of freedom, indicated byarrows:

-   -   two degrees of freedom (XY plane) are provided by the actuation        unit to a distal part of the end-effector,    -   two degrees of freedom (XY plane) are provided by the actuation        unit to a proximal part of the end-effector,    -   one degree of freedom (translation along Z axis) is provided by        the end-effector,    -   one degree of freedom (rotation about Z axis) is provided by the        end-effector.

This architecture allows moving the saw in all required directions tocarry out total knee arthroplasty.

In the example shown here, the support unit (which is optional)comprises two sharp pins 50 connected to the base 1 that are intended tobe in contact with the bone to be treated.

In use, the user pushes the support unit onto the bone to be treated soas to create the partial mechanical link.

Example of Application No. 3—Femoro-Acetabular Impingement (FAI)

FIGS. 9A-9B illustrate a handheld device according to a third embodimentof the invention.

Said device is suitable for treatment of FAI, although not limited tothis specific application.

In the treatment of FAI, the goal is arthroscopic removal of abnormalshapes, looking like osteophytes, at the femoral head-neck junctionand/or the acetabular rim. Currently CT/MRI data are used forpreoperative assessment (determination of a planned volume to beremoved) and X-ray and arthroscopic images are used for intraoperativeorientation. One of the main risks in FAI surgery is over-resection(which would lead to hip instability) or under-resection (which is themost frequent reason for revision surgery). Usually, an arthroscopicapproach using three incisions of one cm each is used. The access to thevolume to be treated is through arthroscopic portals. The surgeon has toavoid injury of hip blood supply, cartilage and/or capsule duringinstrument movement via robotic system; a safe area is thus defined forcompensation with the planning system.

The parts of the device that fulfill the same function as in the devicesdescribed above have the same reference number.

Hence, they will not be described in detail again, apart from theirspecific features.

The handheld device shown in FIGS. 9A-9B comprises an end-effector 2that carries a tool 3; here, the tool 3 is a shaver.

Of course, the user could use another tool.

In particular, the surgical tools that are conventionally used in FAItreatment are high-speed mills and shavers.

This handheld device provides three degrees of freedom:

-   -   two degrees of freedom are provided by the actuation unit 4; in        the embodiment illustrated here, the actuation unit comprises        two spherical five-bar linkages that are synchronously moved by        two motor-gear units. Instead of spherical five-bar linkages one        could also use planar linkages.    -   one degree of freedom (feed motion (z-translation) of the        shaver) is provided by the end-effector 2.

The support unit 5 (which is optional) comprises a cushion 54 c thatprovides a contact area with soft tissues of the patient's body (moreprecisely, with the skin of the hip).

The contact area is as large as possible so as to reduce the pressureapplied to the soft tissues to maintain contact between the cushion 54 cand the tissues.

The cushion 54 c may have a disk shape and be placed on a substantiallyflat part of the patient's body.

Instead, the cushion may have a ring shape allowing it to be placedaround the surgical portal.

The cushion may be fixed to the body of the patient by a strap or by anyother means, e.g. by a suction effect, glue or tape.

In this case, the soft tissues of the patient provide a certain dampingof the partial mechanical link.

Damping may be further provided by a damping or spring characteristicimplemented on the support unit.

Another possibility (in addition to using a pad or vacuum cushion toprovide contact between support unit and soft tissue) is to connect thesupport unit to the trocar which may be used to gain access to thejoint.

Although such a cushion is advantageous in that its fixation to thepatient's body is not invasive, another possibility (in addition to orinstead of) the cushion contacting the soft tissue is to provide acontact with a bone in the vicinity of the part to be treated, e.g. viaat least one pin as described for other kinds of surgery.

The at least one pin can be inserted into the patient's body via theportal through which the tool is introduced or via another portal.

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The invention claimed is:
 1. A surgical system comprising: a surgicaldevice comprising: a base configured to be held in a user's hand, anend-effector for mounting a surgical tool or a tool guide for guidingthe surgical tool, said surgical tool being configured to treat aplanned volume of a part of a patient's body, a motorized actuation unitconnected to said base and said end-effector for moving said surgicaltool or tool guide with respect to the base in order to treat saidplanned volume; and a positioning system configured for positioning saidsurgical device relative to the part to be treated, the positioningsystem comprising: (i) a tracking unit configured to determine in realtime the pose of at least one of the tool, the end-effector and the basewith respect to the part to be treated, (ii) a control unit configuredto: (a) compute in real time a working space of the surgical tool forsaid determined pose, wherein the working space is a union of all posesof the surgical tool relative to the base when a motor of the actuationunit is activated from a minimal value to a maximal value, (b) computein real time the volume that remains to be treated to achieve thetreatment of the planned volume, and (iii) a user interface coupled tothe control unit and configured to display together: (a) the plannedvolume, said planned volume comprising a volume that remains to betreated and, if applicable, an already treated volume, and (b) theworking space of the surgical tool.
 2. The system of claim 1, whereinthe user interface is configured to display the planned volume togetherwith a surface of the part to be treated.
 3. The system of claim 1,wherein the user interface is configured to display the working space asan overlay on the planned volume.
 4. The system of claim 1, wherein thecontrol unit is configured to compute an intersection of the volume thatremains to be treated and the working space of the surgical tool and theuser interface is configured to display said intersection.
 5. The systemof claim 1, wherein the user interface is configured to display in realtime the pose of the tool with respect to the planned volume.
 6. Thesystem of claim 1, wherein the user interface is configured to displayan indication on how the base of the device should be repositioned withrespect to the part to be treated to achieve the treatment of theplanned volume.
 7. The system of claim 1, wherein the control unit isconfigured to stop the actuation unit and/or the tool if said controlunit detects that the current pose of the tool is outside of the plannedvolume.
 8. The system of claim 1, further comprising a holding armconnected to the base of the handheld device and suited to be connectedto a mechanical support.
 9. The system of claim 1, further comprising aplanning system configured to determine a volume to be treated by thetool and, if appropriate, at least one processing parameter of the tool.10. The system of claim 1, wherein a drill guide or a saw guide iscomprised in the end-effector and the tool is a drill or a saw that isintended to be moved within a guiding axis or a guiding plane of saidguide.
 11. The system of claim 1, wherein the surgical tool comprises asaw, a drill, a mill, a shaver or a burr.
 12. The system of claim 1,wherein the tracking unit comprises at least one emitter and at leastone sensor, said at least one emitter being mounted on the base or onthe end-effector and said at least one sensor being adapted to bemounted on the part to be treated.
 13. The system of claim 8, whereinthe mechanical support is an operating table.
 14. The system of claim 1,wherein the surgical tool is configured to remove tissue from thepatient's body.